The present invention relates to a magneto-optical recording medium such as a magneto-optical disk, a magneto-optical tape, a magneto-optical card, and also relates to a recording and reproducing method and an optical head designed for the magneto-optical recording medium.
Research and development on magneto-optical disks have been made as being rewritable optical disks, and some of the magneto-optical disks have been already practically used as external memory designed for computers.
In the magneto-optical disk, a magnetic thin film with perpendicular magnetization is used as a recording medium, and a light is used in recording and reproducing. Thus, compared with a floppy disk or a hard disk wherein a magnetic thin film with in-plane magnetization is used, the magneto-optical disk has larger recording capacity.
However, since recording density of the magneto-optical recording medium is determined by a size of the light beam used in recording and reproducing on the recording medium, there is a limit in increasing memory capacity.
More specifically, when the size of the recording bit and the interval between the recording bits are smaller than the light beam spot diameter, a plurality of recording bits exist including the adjoining recording bits in the light beam spot, which increases noise, thereby presenting the problem that the recording bits cannot be reproduced separately.
In order to increase the recording density, the diameter of the light beam spot can be made smaller by making shorter the wavelength of the laser (light source), or by making larger the number of aperture (NA) of the objective lens so as to have greater angle of conversion.
In order to produce a laser with a shorter wavelength, a semiconductor laser designed for producing a short wavelength laser has been researched and developed. However, the semiconductor laser of this type presents the problem that an output light intensity is too low to be used as a light source for recording and reproducing on and from the magneto-optical disk.
On the other hand, when adapting a method of making larger the NA, it is necessary to maintain the optical axis of the light beam to be converged onto the disk in a perpendicular direction to the surface of the magneto-optical disk. Otherwise, the light beam spot diameter on the recording medium becomes larger on the contrary. Namely, when the NA is made larger, the precision in assembling optical system of the magneto-optical disk device or the restitution of the magneto-optical disk must be more strictly controlled than the conventional model, in order to prevent the problem that the light beam spot diameter becomes larger.
Therefore, at present, a wavelength of the semiconductor laser used in the magneto-optical disk is set in a range of 780-830 nm, and the NA of the objective lens is set in a range of 0.45-0.55. Accordingly, a light beam spot diameter on the recording medium is set in a range of 1.7-2.0 xcexcm.
In accordance with the light beam spot diameter, a truck pitch of the magneto-optical disk, i.e., an interval between the recording bits in a radial direction of the magneto-optical disk is set in a range of 1.4-1.6 xcexcm.
If the light beam spot diameter is made smaller than the above, it is necessary to prevent the crosstalk which disturbs information recorded on the adjoining trucks. In order to prevent the crosstalk, a compensating circuit for carrying out a special waveform processing is required to be separately provided, thereby presenting the problem that the magneto-optical disk device becomes complicated.
In the case of carrying out an overwriting operation by the magnetic field modulation onto the magneto-optical disk, in order to obtain a sufficient size of the magnetic field, the magnetic field generation mechanism is required to be set close to the magneto-optical disk. Moreover, the magnetic field cannot be modulated at high speed.
In order to counteract the above problems, as disclosed in the Japanese Laid Open Patent Publication No. 62-175948 (Tokukaisho 175948/1987), an overwriting method by the light intensity modulation in which using a magneto-optical recording medium of a double layer structure of a recording layer composed of a magnetic thin film with perpendicular magnetization and a recording subsidiary layer, an overwriting operation is carried out by modulating only the laser power has been proposed.
In the above overwriting method by the light intensity modulation, when carrying out an overwriting operation, the magnetization direction of the recording subsidiary layer also changes. Thus, whenever an overwriting operation is to be carried out, it is necessary to arrange the magnetization direction of the recording subsidiary layer. When adapting the above method, not only the recording magnetic field generating mechanism, an initialization-use magnetic field generating mechanism is also required, thereby presenting the problems that the magneto-optical disk device becomes larger in size and that the manufacturing costs thereof increases.
An object of the present invention is to provide a magneto-optical recording medium whereon a high density recording is permitted and to provide a method of recording and reproducing information on and from the recording medium and also to provide an optical head used in recording and reproducing information on and from the magneto-optical medium.
In order to achieve the above object, the magneto-optical recording medium of the present invention is characterized by comprising:
a base having a property that a light can be transmitted therethrough;
a readout layer formed on said base, which has in-plane magnetization at room temperature, whereas, a transition occurs from in-plane magnetization to perpendicular magnetization as temperature rises; and
a recording layer formed on the readout layer, for recording thereon information magneto-optically. According to the above arrangement, in reproducing, when a light beam is projected onto the readout layer, the temperature distribution of the light projected portion shows a Gaussian distribution, and thus only the temperature of the central portion which is smaller than the light beam diameter is raised.
As the temperature rises, a transition occurs in the light projected portion form in-plane magnetization to perpendicular magnetization. Here, by the exchange coupling force exerted between the readout layer and the recording layer, the magnetization direction of the readout layer is arranged in the magnetization direction of the recording layer.
When a transition occurs from in-plane magnetization to perpendicular magnetization in the portion having a temperature rise, polar Kerr effect is shown only in the portion, thereby reproducing information based on the light reflected therefrom.
When a light beam is shifted so as to reproduce the next recording bit, the temperature of the previously reproduced potion is cooled off, and thus a transition occurs from perpendicular magnetization to in-plane magnetization in the portion and the polar Kerr effect is no longer shown in the portion. This means that the magnetization recorded on the recording layer ifs not readout by being masked by the in-plane magnetization of the readout layer. Therefore, information is no longer reproduced from the spot having the temperature drop and thus interference by signals from the adjoining bits, which is the case of generating noise is eliminated.
As described, in the above arrangement, only the portion having a temperature rise above predetermined temperature is subjected to reproduction. Therefore, the reproduction of a smaller recording bit is enabled compared with the conventional model, thereby permitting an improvement in the recording density.
Furthermore, by adapting, for example, GdFeCo of rare-earth transition metal alloy for the readout layer, the readout layer in which a transition occurs quickly from in-plane magnetization to perpendicular magnetization can be achieved. As a result, noise generated in reproducing can be reduced, thereby providing a magneto-optical recording medium which permits high density recording.
Furthermore, in order to achieve the above object, a recording and reproducing method of the present invention for recording and reproducing information on and from a recording medium comprising a base having a property that a light can be transmitted therethrough, a readout layer formed on the base, which has in-plane magnetization at room temperature, and a transition occurs from in-plane magnetization to perpendicular magnetization as temperature rises, and a recording layer formed on the readout layer, for recording thereon information magneto-optically, wherein the readout layer made of rare-earth transition metal alloy is set so as to have its compensation temperature outside the range of room temperaturexe2x80x94Curie temperature, and the content of the rare-earth metal is greater than the maximum amount at compensating composition, is characterized in that the magnetization direction of the recording layer is reversed by projecting a laser beam whose intensity is changed between a relatively high power of the first power and a relatively low power of the second power while a constant magnetic field for magnetizing the readout layer is being applied, so as to record information, and in that the magnetization in an area smaller than the laser spot diameter of the readout layer is changed to perpendicular magnetization by projecting a laser beam having a lower power lower than the first laser beam, and the sub-lattice magnetization in the area having a perpendicular magnetization of the readout layer is arranged in a stable direction with respect to the sub-lattice magnetization of the recording layer, thereby reproducing information from the area having perpendicular magnetization of the readout layer.
In the above arrangement, a high density recording and reproducing is permitted on and from the magneto-optical disk having the above configuration.
In order to achieve the above object, the optical head of the present invention comprises a semiconductor laser, a collimator lens for converging a laser beam generated from the semiconductor laser into a parallel beam, and an objective lens for converging a light beam onto the readout layer, wherein the aperture of the objective lens is set smaller than the diameter of the light beam.
In the above arrangement, the light beam spot diameter by the main robe can be set smaller, the information recorded at high density can be reproduced. Moreover, when the magneto-optical recording medium is adapted, the effect from the crosstalk due to the side robe can be avoided.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.